Thermosetting polymers can not be converted into a molten state or dissolved in solvents. Although these materials offer enhanced mechanical and thermal properties over the thermoplastics, they cannot be readily processed into finished products using processing techniques, commonly used in the case of thermoplastics. Similarly, the properties of the thermoplastics cannot be significantly enhanced after converting the resins into finished products since there is no scope to modify the polymer structure chemically after the polymerization is completed.
In certain thermosetting polymers, reactive groups are introduced in the backbone. These polymers are usually in the form of lattices that are further crosslinked either thermally or by addition of functional groups like isocyanates, amines or metal ions. These resins attain their desired properties i.e., insolubility in most organic solvents, good water resistance and hardness by network formation and are used as coatings. (Van E.S.J.J. in Polymeric Dispersions Principles and Applications. Asua, J. M. (Ed), Kluwer Publishers, 1997, p. 451; Ooka, M. Ozawa. H. Progress in Organic Coatings, vol. 23, 1994, p. 325). The need for polymers which are water soluble and thermally fusible and which could be later converted into products having enhanced mechanical/thermal/solvent resistance properties is increasing with growing applications of polymers in different fields.
Water insoluble molecules become water-soluble on treatment with aqueous solutions of cyclodextrins or similar host molecule. The inclusion phenomenon leads to significant changes of solution properties and reactivity of the guest molecule. The formation of inclusion complexes of hydrophobic monomers with β-cyclodextrin or its derivatives has been reported. (Storsberg, J., Ritter, H. Macromolecular Communications 21, 230, 2000. Jeromin, J. Ritter, H. Macromolecular Communications 19, 377, 1998. Jeromin, J. Noll, O. Ritter, H. Macromolecular Chemistry & Physics, 199, 2641-1998. Glockner, P. Ritter, H. Macromolecular Rapid Communications, 20, 602, 1999). It has been established that the reactivity ratios of complexed monomers differ significantly from those of the un-complexed monomers.
Complexes of cyclodextrin have been investigated in the past. U.S. Pat. No. 4,906,488 describes these for the release of permeants to the outside hosts. Similarly, U.S. Pat. No. 5,258,414 describes encapsulation of blowing agents into cyclodextrins and incorporation of the complexes into thermoplastics for delayed release of the blowing agents. U.S. Pat. No. 5,268,286 describes the method for polymerization of biocatalysts on polymers. Similarly, U.S. Pat. No. 5,290,831 describes the use of cyclodextrins for stabilization of the polymerization initiators as to regulate the polymerization in a desirable manner.
U.S. Pat. No. 6,180,739 describes polymerizable cyclodextrin derivatives for applications in dental resins, which adhere strongly to dentin. The said patent covers polymerizable cyclodextrin derivatives wherein cyclodextrin is reacted with a large excess of monomer so that each cyclodextrin molecule is covalently linked to a large number of polymerizable groups. The compositions are useful in dental and industrial formulations. Another feature of this invention is the presence of functional groups in the polymer structure, which can form hydrogen bonds, ionic bonds, and Van der Waal interactions with the appropriate substrate so as to enhance adhesion. The invention also covers initiators, which are encapsulated in cyclodextrin cavity. The cyclodextrin is an integral part of the polymer structure and has a functional role in application.
The said patent deals with functionalized polymers containing cyclodextrin wherein cyclodextrin are covalently bonded to a monomer that the polymer structure contains cyclodextrin. Thus, the subject matter of the invention covered by this patent is a highly substituted or derivatized cyclodextrin containing unsaturated groups. Another feature if this invention is presence of a functionalized group in the polymer structure which can form hydrogen bonds, ionic bonds, Van der Waal interactions with appropriate substrate so as to enhance adhesion. The invention also covers photo sensitive initiators, which are encapsulated in cyclodextrin. The invention thus deals with complexes of thermal initiators encapsulated in cyclodextrin derivatives. In many cases, the functional sites such as carboxyl groups present in the polymer are bridged using calcium or other di-cationic metals so as to provide cross linking.
The subject matter of our invention deals with the encapsulation of the cross linker which can contain more than one unsaturated groups. It may be noted here that the interaction between the cross linker and cyclodextrin exploited in this work is non-covalent in nature. As a result of this complexation, vinyl groups present in the cross linker but encapsulated in the cyclodextrin cavity do not take part in polymerization and prevent cross linking. Also, cyclodextrin can be removed after polymer has been formed and is not a part of the resulting polymer structure after the second stage of cross linking is effected either by thermal or photo chemical polymerization. It may be further mentioned that the initiators used by us are in their free form and are not encapsulated in cyclodextrin.
In our copending application No. NCL-28-2002 (PCT Application No. PCT/IB03/03593) we have described the preparation of inclusion complexes of cyclodextrins with monomers containing multiple unsaturations. Polymerization of these complexes gives rise to soluble homopolymers containing unsaturated sites, which can be further crosslinked. But applications of homopolymers of monomers having multiple unsaturations are limited. Copolymerization of different monomers gives rise to tailor made materials for a wide range of applications. Depending upon the composition of the comonomers, either hydrophilic, hydrophobic or amphiphilic polymers can be synthesized. If unsaturated groups are incorporated into these copolymers, they can then be converted into films, membranes or micro or nanoparticles and can be crosslinked in a second step. Such polymers would find applications in electronics, photoresists, controlled release delivery systems, micro electro mechanical systems (MEMS) etc.